> On 3 Dec 2017, at 04:11, Matthew Johnson via swift-evolution 
> <swift-evolution@swift.org> wrote:
> Sent from my iPad
> On Dec 2, 2017, at 7:40 PM, Chris Lattner <clatt...@nondot.org 
> <mailto:clatt...@nondot.org>> wrote:
>> On Dec 2, 2017, at 2:13 PM, Matthew Johnson <matt...@anandabits.com 
>> <mailto:matt...@anandabits.com>> wrote:
>>>> For all those reasons, we really do need something like AnyObject dispatch 
>>>> if we care about working with dynamically typed languages.  The design I’m 
>>>> suggesting carefully cordons this off into its own struct type, so it 
>>>> doesn’t infect the rest of the type system, and is non-invasive in the 
>>>> compiler.
>>> I am quite familiar with dynamic languages and agree that this is necessary 
>>> if we are going to fully open up access to these languages from Swift.
>> Ok, then it appears you agree that something like anyobject dispatch is 
>> necessary for effective dynamic language interop.
>>>>> I strongly urge you to reconsider the decision of that dynamic members 
>>>>> must be made available with no indication at usage sites.  An indication 
>>>>> of dynamic lookup at usage sites aligns very well (IMO) with the rest of 
>>>>> Swift (AnyObject lookup aside) by calling attention to code that requires 
>>>>> extra care to get right.
>>>> I don’t understand this.  The proposal is fully type safe, and this 
>>>> approach is completely precedented by AnyObject.  Swift’s type system 
>>>> supports many ways to express fallibility, and keeping those decisions 
>>>> orthogonal to this proposal is the right thing to do, because it allows 
>>>> the author of the type to decide what model makes sense for them.
>>> Allowing the author of the type to choose whether the mechanism is hidden 
>>> or visible is exactly what I don’t want to allow.  I think you have the 
>>> right design regarding types and semantics - the author chooses.  But I 
>>> don’t want these calls to look like ordinary member lookup when I’m reading 
>>> code.  
>>> They inherently have a much greater chance of failure than ordinary member 
>>> lookup.  Further, authors are likely to choose immediate traps or nil IUO 
>>> as failure modes as forcing users to deal with Optional on every call is 
>>> likely to be untenable.  I believe this behavior should be represented by 
>>> some kind of syntax at the usage site.  I don’t believe it is an undue 
>>> burden.  It would make the dynamic lookup semantic clear to all readers and 
>>> would help to discourage abuse.
>> I believe that adding explicit syntax would be counterproductive to your 
>> goals, and would not make dynamic lookup syntax more clear.  I assume that 
>> you would also want the same thing for DynamicCallable too, and operator 
>> overloads, subscripts, and every other operation you perform on these 
>> values, since they all have the exact same behavior.
>> If we required some syntax even as minimal as “foo.^bar” and "baz^(42)”, 
>> that change would turn this (which uses runtime failing or IUO return values 
>> like AnyObject):
>>      let np = Python.import("numpy")
>>      let x = np.array([6, 7, 8])
>>      let y =  np.arange(24).reshape(2, 3, 4)
>>      let a = np.ones(3, dtype: np.int32)
>>      let b = np.linspace(0, pi, 3)
>>      let c = a+b
>>      let d = np.exp(c)
>>      print(d)
>> into:
>>      let np = Python.import("numpy")
>>      let b = np^.array^([6, 7, 8])
>>      let y =  np^.arange^(24)^.reshape^(2, 3, 4)
>>      let a = np^.ones^(3, dtype: np^.int32)
>>      let b = np^.linspace^(0, pi, 3)
>>      let c = a+^b
>>      let d = np^.exp^(c)
>> This does not improve clarity of code, it merely serves to obfuscate logic.  
>> It is immediately apparent from the APIs being used, the API style, and the 
>> static types (in Xcode or through static declarations) that this is all 
>> Python stuff.  
> It may be immediately apparent when the types involved are obviously dynamic, 
> such as in this example where Python.import is explicitly used.  However, my 
> concern is less about the intended use case of dynamic language interop than 
> I am that this feature will be generally available to all types in Swift.  
> This is big change from AnyObject dispatch.  It opens up the dynamism to 
> types and contexts that are not necessarily obviously using dynamic lookup, 
> callable, etc.  Maybe this won’t turn out to be a problem in practice but I 
> still think it’s a legitimate concern.

If dynamism if restricted to subclasses of a DynamicObject type, like Xiaodi 
suggested earlier, then we can protect ourselves from this dynamic dispatch 
being generally available to all types in Swift.

>> When you start mixing in use of native Swift types like dictionaries 
>> (something we want to encourage because they are typed!) you end up with an 
>> inconsistent mismash where people would just try adding syntax or applying 
>> fixits continuously until the code builds.
>> Beyond that, it is counterproductive to your goals, because it means that 
>> people are far less likely to use to use optional returns.  Doing so (which 
>> produces a safer result) would cause a double tax in syntax, and would be a 
>> confusing jumble.  I can’t bring myself to do the whole example above, one 
>> line - just converting member lookup syntax but not callable syntax - would 
>> end up:
>>      let y =  np^.arange?^(24)^.reshape^?(2, 3, 4)
>> If you made DynamicCallable also return optional it would be:
>>      let y =  np^.arange?^(24)?^.reshape^?(2, 3, 4)!
>> or something.  This is such madness that no one would do that.
> Yes, I agree.  The interaction with optional chaining makes it unworkable.  I 
> hadn’t thought that all the way through.  Thank you for indulging in the 
> discussion about this idea.  
> I’m uncertain what the right answer is.  I’m still not really comfortable 
> with opening up dynamic lookup to any user-defined type without some way to 
> indicate to readers that dynamic lookup is happening in a piece of code.  
> Maybe there is a less localized annotation that would indicate dynamic lookup 
> is in effect for a larger chunk of code.

I think that making dynamic calls syntactically different to readers is going 
too far in the direction of safety. Plus, it makes the language inconsistent as 
we already have AnyObject dispatch with exactly the same syntax. But I 
understand why you would want it if *any* type could end up being conformed to 
a dynamic lookupable/callable protocol. Like said above, I think that a 
DynamicObject type is enough protection to not bother making the syntax heavier 
at the point of use.

> One idea that hasn’t been explored yet is introducing a dynamic lookup 
> effect.  That would provide clean syntax at the expression level while still 
> making it clear that dynamic lookup is happening.  This probably isn’t the 
> right approach as it would be viral and wouldn’t even indicate that the code 
> in a specific function body even contains a dynamic lookup.  I mention it 
> mostly to broaden the conversation and thought space about what kind of 
> approach might address my concerns without cluttering up expressions.
> Another, potentially more viable approach would be to use an approach similar 
> to try and the proposed async of a statement modifier (possibly also allowed 
> at the expression level as with try and async).  This could be used in 
> conjunction with an effect as mentioned above but could also be used 
> independently even though there isn’t a current precedent for that.  Your 
> example written with this approach might be:
>       let np = Python.import("numpy")
>       let b = dynamic np.array([6, 7, 8])
>       let y =  dynamic np.arange(24).reshape(2, 3, 4)
>       let a = dynamic np.ones(3, dtype: np.int32)
>       let b = dynamic np.linspace(0, pi, 3)
>       let c = dynamic a + b
>       let d = dynamic np.exp(c)
> Note: `dynamic` is just a straw man here.  This is obviously a lot of 
> boilerplate repetition of the same modifier.  I would also be perfectly happy 
> allowing this annotation to apply to a block of code or even a whole function 
> body.  Then it might be:
> dynamic {
>       let np = Python.import("numpy")
>       let b = np.array([6, 7, 8])
>       let y = np.arange(24).reshape(2, 3, 4)
>       let a = np.ones(3, dtype: np.int32)
>       let b = np.linspace(0, pi, 3)
>       let c = a + b
>       let d = np.exp(c)
> }
> The most obvious objection to this is that it introduces nesting and does so 
> in a way that only very indirectly influences control flow (through the 
> potential for dynamic failure).
> My objective certainly isn’t to make code ugly and obfuscate logic.  It is 
> simply to make the usage of dynamic features that are prone to failure at 
> runtime immediately clear to a reader.  This should be as lightweight as 
> possible while still providing valuable information to the reader.
>>>> Swift already has a dynamic member lookup feature, "AnyObject dispatch" 
>>>> which does not use additional punctuation, so this would break precedent.
>>> I would prefer if dynamic lookup were visible with AnyObject as well.  For 
>>> that reason I don’t believe it makes a good precedent to follow.  In fact, 
>>> I would prefer to see us go the other direction and perhaps even consider 
>>> revising dynamic lookup syntax for AnyObject in the future.
>> This is definitely not going to happen.  The change Doug mentioned is to 
>> have AnyObject lookup return optional instead of IUO, which forces ? on the 
>> clients.  Adding other syntax (like you’re suggesting) is certainly not 
>> going to happen.
>> The entire point of AnyObject dispatch is to improve syntactic elegance and 
>> clarity of code using it.  There is no other reason to exist.  Making code 
>> that uses it syntactically onerous completely defeats the point of having it 
>> in the first place, as I’ve mentioned before.
> I agree.  The interaction with optional chaining is significantly more 
> onerous than I had considered.  I should have worked through an example on my 
> own.  I do hope you will consider a less localized approach to usage-site 
> annotation though.
>> Furthermore, your premise that Swift does not have invisibly failable 
>> operations is plainly wrong.  Array subscript and even integer addition can 
>> fail.  
> I am well aware of these behaviors.  The difference IMO is that programmers 
> tend to be well aware of these preconditions even if they also often choose 
> to ignore them.  Dynamic lookup will not be so clear.  This is especially 
> true if people use it with types that also have an API available through 
> static lookup.
>> Even the behavior of AnyObject was carefully designed and considered, and 
>> were really really good reasons for it returning IUO.
> I am not trying to call into question the choices made in the past.  Swift 
> wouldn’t be the great language with a bright future that it is today without 
> an incredibly successful migration of a large user base from Objective-C to 
> Swift.  This is a huge accomplishment and couldn’t have happened without 
> making really good decisions about some really hard tradeoffs.
>> -Chris
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